Huntington disease (HD) is an autosomal-dominant neurodegenerative disorder that primarily affects medium spiny striatal neurons (MSN). The symptoms are choreiform, involuntary movements, personality changes and dementia. HD is caused by a CAG repeat expansion in the IT15 gene, which results in a long stretch of polyglutamine (polyQ) close to the amino-terminus of the HD protein huntingtin (Htt). Mutant Htt (mHtt) has effects both in the cytoplasm and in the nucleus. Full-length Htt is cleaved by proteases in the cytoplasm, leading to the formation of cytoplasmic and neuritic aggregates. mHtt also alters vesicular transport and recycling, causes cytosolic and mitochondrial Ca2+ overload, triggers endoplasmic reticulum stress through proteasomal dysfunction, and impairs autophagy function, increasing neuronal death susceptibility. N-terminal fragments containing the polyQ strech translocate to the nucleus where they impair transcription and induce neuronal death.